High strength, fine grained spinel for window applications, methods of manufacture thereof and articles comprising the same

ABSTRACT

Disclosed herein is a spinel article. The article comprises a spinel material, wherein the spinel material has a monomodal grain size distribution with average grain sizes of less than or equal to about 15 micrometers, and a biaxial flexural strength of greater than or equal to about 300 megapascals when measured by a ring-on-ring flexural test as per ASTM Standard C1499-08. Disclosed herein too is a spinel article manufactured by a method comprising calcining a spinel powder; milling the powder in a milling medium; granulating the powder; screening the powder to a mesh size of about 40 to about 200 mesh; pressing the powder to form an article; burning out organics from the article; sintering the article; and hot isostatically pressing the article.

BACKGROUND OF THE INVENTION

This disclosure relates to high strength fine-grained spinel for windowapplications, methods of manufacture thereof and to articles comprisingthe same.

High strength, high hardness, transparent materials with low opticalscatter are needed for transparent armor such as Humvee windows, and asinfrared windows for missile radomes. Currently ballistic glass bondedto plastic is being used for transparent armor while single crystalAl₂O₃ (sapphire) is being used as a missile dome material. Ballisticglass suffers with issues related to its hardness and ballistic stoppingpower per unit of weight. Sapphire domes are expensive since there is asubstantial amount of machining required of the single crystal boule tomake suitable dome shapes. It is therefore desirable to find a materialother than glass and sapphire that can be used.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed herein is a spinel article. The article comprises a spinelmaterial, wherein the spinel material has a monomodal grain sizedistribution with average grain sizes of less than or equal to about 15micrometers, and a biaxial flexural strength of greater than or equal toabout 300 megapascals when measured by a ring-on-ring flexural test asper ASTM Standard C1499-08.

Disclosed herein too is a spinel article manufactured by a methodcomprising calcining a spinel powder; milling the powder in a millingmedium; granulating the powder; screening the powder to a mesh size ofabout 40 to about 200 mesh; pressing the powder to form an article;burning out organics from the article; sintering the article; and hotisostatically pressing the article.

Disclosed herein too is a method comprising calcining a spinel powder;milling the powder in a milling medium; granulating the powder;screening the powder through a mesh size of about 40 to about 200 mesh;pressing the powder to form an article; burning out organics from thearticle; sintering the article; and hot isostatically pressing thearticle.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a depiction of the process for producing the spinel materialdisclosed herein;

FIG. 2 is a graph showing the specular transmission in the wavelengthrange of 500 nanometers to 900 nanometers of a 9.6 millimeter thickspinel sample that was prepared in the Example disclosed herein;

FIG. 3 is a scanning electron micrograph of the microstructure in thespinel material prepared in the Example disclosed herein;

FIG. 4 is a Weibull plot of the biaxial flexural strength measured fromspinel samples that were prepared in the Example disclosed herein; and

FIG. 5 shows a comparison of the biaxial strength measured from thespinel material disclosed herein with the biaxial strength measured fromthe state-of-the-art hot pressed spinel material acquired from a vendor.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein is a spinel that has a monomodal grain sizedistribution with an average grain size of less than 15 micrometers,with a biaxial flexural strength (when measured by a ring-on-ringflexural test as per ASTM Standard C1499-08) of greater than or equal toabout 300 megapascals (MPa) and with specular transmittance of greaterthan or equal to about 75% for wavelengths of 632 nanometers to 800nanometers for a sample having a thickness of about 5 to about 10millimeters. In an exemplary embodiment, the spinel has a speculartransmittance of greater than or equal to about 80 percent (%) forwavelengths of 632 nanometers to 800 nanometers for a sample having athickness of about 9.6 millimeters.

Spinel having a biaxial strength of greater than or equal to about 300MPa and a specular transmittance of at least 80% allows for betterperformance of the spinel material as transparent armor and in radomes.Higher strength makes the material more capable of withstanding stressesin service, and high specular transmittance makes the material good forwindows, giving a high signal to noise ratio of unscattered light toscattered light for a user/detector that is to be protected by thewindow. Spinel having a grain size of less than or equal to about 15micrometers is easier to polish without grain pull-out pitting whencompared to a larger grained spinel, thus reducing polishing costs. Inaddition, biaxial strengths of greater than or equal to about 300 MPamake the material more robust as a window to withstand stresses duringservice. Spinel may be used in lieu of glass and sapphire in transparentarmor when its specular transmittance of light having wavelengths of 632nanometers (nm) to 800 nm can be increased to greater than or equal toabout 80% for a part having a thickness of greater than or equal toabout 9.6 millimeters.

The spinel can have a variety of different compositions. Examples ofspinels are spinel (MgAl₂O₄), MgGa₂O₄, gahnite (ZnAl₂O₄), franklinite(Fe,Mn,Zn)(Fe,Mn)₂O₄, chromite (Fe.Mg)Cr₂O₄, magnetite (Fe₃O₄),hercynite (FeAl₂O₄), ulvöspinel (TiFe₂O₄), jacobsite (MnFe₂O₄),trevorite (NiFe₂O₄), ringwoodite (SiMg₂O₄), ZnGa₂O₄, SiMg₂O₄, or thelike, or a combination comprising at least one of the foregoing spinels.In an exemplary embodiment, the spinel is MgAl₂O₄.

It is desirable for the spinels to be optically transparent. Examples ofoptically transparent spinels are MgAl₂O₄, MgGa₂O₄, ZnAl₂O₄, ZnGa₂O₄,SiMg₂O₄, or a combination comprising at least one of the foregoingspinels. In one embodiment, the spinels can be tinted if desired. Acoloring or tinting agent comprising metal ions such as iron, manganese,chromium, cobalt, and the like, may be added to the spinel window.

FIG. 1 depicts an exemplary embodiment for manufacturing the spineldisclosed herein. The spinel is generally produced as a powder prior tostarting the process to manufacture the dense body having the desiredproperties. The spinel powder may comprise small amounts of contaminantsand trace amounts of other elements. In general it is desirable for thespinel powder to have metallic contaminants each of less than 5 partsper million (ppm) by weight. In one embodiment, it is desirable to haveless than 2 ppm of any of the following elements: chromium, cobalt,nickel, copper, lead, titanium, vanadium and zinc. It is desirable forthe spinel powder to have less than or equal to about 50 ppm ofchlorine, less than or equal to about 50 ppm of sulfur, less than orequal to about 30 ppm of calcium, less than or equal to about 30 ppm ofsodium, less than or equal to about 100 ppm silicon, less than or equalto about 12 ppm manganese, and less than or equal to about 30 ppm iron.

In an exemplary embodiment, it is desirable to have less than or equalto about 10 ppm of chlorine, less than or equal to about 20 ppm ofsulfur, less than or equal to about 15 ppm calcium, less than or equalto about 15 ppm sodium less than or equal to about 10 ppm potassium;less than or equal to about 50 ppm silicon, less than or equal to about6 ppm manganese and less than or equal to about 15 ppm iron.

The powder generally has a surface area of about 20 to about 60 squaremeters per gram (m²/gm). In one embodiment, the powder has a surfacearea of about 30 to about 50 m²/gm. In another embodiment, the powderhas a surface area of about 35 to about 45 m²/gm.

The average particle size of the powder is about 10 to about 200nanometers (nm). In one embodiment, the average particle size of thepowder is about 20 to about 150 nm. In another embodiment, the averageparticle size of the powder is about 30 to about 120 nm.

The powder is subjected to the process depicted in the FIG. 1. Thepowder is calcined to about 600 to about 900 degrees centigrade (° C.)to remove any residual organics that may be present in the powderfollowing the production of the powder. In one embodiment, the powder iscalcined to about 650 to about 850° C. In another embodiment, the powderis calcined to about 700 to about 800° C.

The powder is then subjected to milling and/or ultrasonication in orderto reduce the particle sizes. Suitable examples of milling are ballmilling, attrition milling, vibratory milling, jet milling, or acombination comprising at least one of the foregoing processes. Ballmilling is an exemplary method by which the particle sizes are reduced.The milling is conducted in a milling medium for about 12 to about 48hours. The milling medium comprises a liquid containing at least one ofa dispersant, a binder, an optional plasticizer and a metal salt wherethe metal is from Group I of the periodic table. The liquid can containcombinations of the dispersant, the binder, the optional plasticizer andthe metal salt where the metal is from Group I of the periodic table. Inone embodiment, the milling medium comprises a liquid that comprises adispersant, a binder, an optional plasticizer and a metal salt where themetal is from Group I of the periodic table.

The liquid can be an organic solvent or water. In one exemplaryembodiment, the liquid is water. In another exemplary embodiment, themilling medium is in the form of a solution. In yet another exemplaryembodiment, the milling medium is in the form of a suspension.

The dispersant can be an acid, a pre-neutralized acid, or a base.Examples of suitable acidic dispersants are carboxylic acids includingcitric acid, succinic acid, polyacrylic acid, acetic acid, or the like,or a combination comprising at least one of the foregoing acids.Examples of pre-neutralized acids are diammonium citrate, ammoniumcitrate tribasic, ammonium polyacrylate, ammonium acetate, or the like,or a combination comprising at least one of the foregoingpre-neutralized acids. Examples of suitable basic dispersants areammonium hydroxide, tetramethyl ammonium hydroxide, or the like, or acombination comprising at least one of the foregoing bases.

The dispersant is added to the liquid in an amount of about 1 to about 5weight percent (wt %), based on the total weight of the powder. In oneembodiment, the dispersant is added to the liquid in an amount of about2 to about 4 wt %, based on the total weight of the powder.

The binder can be an organic polymer. The organic polymer can be athermoplastic polymer, a thermosetting polymer, a blend of thermoplasticpolymers, a blend of thermosetting polymers or a blend of thermoplasticpolymers with thermosetting polymers. The organic polymer can be ahomopolymer, a copolymer, a star block copolymer, a block copolymer, arandom copolymer, an alternating block copolymer, an ionomer, adendrimer, a polyelectrolyte, or the like, or a combination comprisingat least one of the foregoing copolymers. It is generally desirable forthe organic polymer to be compatible with the liquid used for themilling. In one embodiment, it is desirable for the organic polymers tobe water soluble.

Examples of suitable organic polymers that can be used as a binder arepolyacrylamide, polyvinylalcohol, polyvinylpyrrolidone, polyethyleneglycol, hydroxyethylcellulose, methylcellulose, polyethyleneimine, orthe like, or a combination comprising at least one of the foregoingorganic polymers.

The binder can be used in an amount of about 0.5 to about 5 wt %, basedon the total weight of the powder. In one embodiment, the binder ispresent in the milling medium in an amount of about 1 to about 4 wt %,based on the total weight of the powder. In another embodiment, thebinder is present in the milling medium in an amount of about 1.5 toabout 3 wt %, based on the total weight of the powder.

As noted above, the milling medium comprises an optional plasticizer. Itis generally desirable for the plasticizer to plasticize the organicpolymer. Examples of suitable plasticizers are glycerin, ethyleneglycol, tetraethylene glycol, dibutyl phthalate, or the like, or acombination comprising at least one of the foregoing plasticizers.

The plasticizer is generally used in an amount of about 0.1 to about 5wt %, based on the total weight of the powder. In one embodiment theplasticizer is used in an amount of about 0.5 to about 4 wt %, based onthe total weight of the powder. In another embodiment the plasticizer isused in an amount of about 1 to about 3 wt %, based on the total weightof the powder.

As noted above, the milling medium comprises a metal salt as a sinteringaid. The metal in the metal salt is from Group I of the periodic table.Suitable metals from the Group I are lithium, sodium, potassium andcesium. Examples of suitable sintering aids are hydroxides, carbonates,oxalates, acetylacetonates, acetates and nitrates of lithium, cesiumsodium or potassium. Combinations of the aforementioned sintering aidscan also be used. The milling medium generally contains lithium in anamount of about 2 to about 100 parts per million, based on the totalweight of the powder. In one embodiment, the milling medium generallycontains lithium in an amount of about 5 to about 50 parts per million,based on the total weight of the powder. In an exemplary embodiment, thelithium is added in an amount of about 10 parts per million, based onthe total weight of the powder.

The powder content in the milling medium is about 5 to about 30 volumepercent. In one embodiment, the volume of the powder in the millingmedium is about 12.5 to about 22.5 volume percent. In one embodiment,the volume of the powder in the milling medium is about 15 to about 20volume percent.

Following the milling operation, any polymer particles that have beengenerated during the milling operation are extracted from the millingmedium. The polymeric particles can be removed from the suspension by aprocess that includes centrifugation and skimming or separatoryfunneling, gravitational floatation and skimming, or separatoryfunneling, filtration, or the like, or a combination comprising at leastone of the foregoing processes.

The dispersed powder in the milling medium is then subjected togranulation to produce a granulated powder. Granulation is generallyconducted by a process including pan drying and crushing using a mortarand pestle, spray drying, freeze granulation, or the like, or acombination comprising at least one of the foregoing processes.

The granulated powder is then subjected to screening to obtain powdersthat have an average particle size of less than about 40 mesh to about200 mesh. The granulated powder is generally sieved to obtain powdersthat have an average particle size of less than about 40 mesh to about200 mesh. In one embodiment the powders after sieving have an averageparticle size of less than about 50 mesh to about 170 mesh. In anotherembodiment the powders after sieving have an average particle of lessthan about 60 mesh to about 120 mesh.

The granulated powder is then subjected to pressing using an isostaticpressure of about 140 megapascals (MPa) to about 420 MPa to form anarticle having a desired shape. In one embodiment, the granulated powderis subjected to a pressure of about 170 to about 350 MPa. In anotherembodiment, the granulated powder is subjected to a pressure of about200 to about 300 MPa to form a desired article. The powder may beuniaxially pre-pressed in a die, and/or placed in a mold or a die priorto applying the isostatic pressure. In one embodiment, the article is amonolith.

Alternatively, in order to manufacture an article from the fine powder,processes such as slip casting, pressure filtration, gel casting,injection molding, freeze casting, extrusion, and tape casting followedby lamination can be used. Following the manufacturing of a desiredarticle, the organic polymers and other organic binders are burned outof the article in an air and/or an oxygen atmosphere over the course ofabout 2 to about 7 days. The various parts of the desired articlegenerally have a thickness of about 1 to about 18 millimeters. In oneembodiment, the various parts of the desired article generally have athickness of less than or equal to about 16 millimeters. In anotherembodiment, the various parts of the desired article generally have athickness of less than or equal to about 14 millimeters.

The burned-out article is sintered to a temperature of about 1100° C. toabout 1350° C. for a time period of about 0.5 hours to about 30 hours.The sintering is conducted under a vacuum or alternatively in thepresence of wet hydrogen, dry hydrogen, or oxygen. Sintering results ina body with closed porosity prior to hot isostatically pressing, with adensity in the range of about 94 to about 99.99% of the theoreticalvalue that can be attained. The density value for the pore-free spinelis 3.58 grams per cubic centimeter. In one embodiment, the sinteringproduces a density of about 96 to about 99% of the theoretical valuethat can be attained. In one embodiment, the sintering produces adensity of about 96.5 to about 97.5%, of the theoretical value that canbe attained. An article with closed porosity can also be prepared by hotpressing or spark plasma sintering. Alternatively, an article with someopen porosity can be directly hot isostatically pressed in a “can”material such as platinum or other refractory metal.

The article is then subjected to hot isostatic pressing. In oneembodiment, the article is subjected to hot isostatic pressing to atemperature of about 1300 to about 1600° C. and a pressure of about 15to about 60 kilograms per square inch (ksi). In another embodiment, thearticle is subjected to hot isostatic pressing to a temperature of about1400 to about 1500° C. and a pressure of about 25 to about 50 kilogramsper square inch (ksi).

The article is then subjected to polishing and tested. The biaxialflexural strength was measured by a ring-on-ring test method asprescribed by ASTM C1499-08. The samples have a diameter of 14millimeters and have a thickness of 0.8 mm. The articles are supportedby a 12.5 millimeter ring and loaded from the top with a 6.25 millimeterring. That produces a state of uniform bi-axial tensile stress in thecenter part of the bottom surface of the sample, where the fractureoriginates.

Fracture in ceramics initiates from defects in the microstructurebecause stress is concentrated in those areas. Defects can be due toprocessing such as porosity, large grain agglomerates, or foreignmaterial incorporated as a contaminant or they can be caused by samplepreparation, such scratch marks produced during grinding. In thering-on-ring test, the side of the sample that is in tension is normallypolished to a mirror finish in order to minimize grinding defects.

The articles produced by the aforementioned method have advantageousproperties. The articles have a monomodal grain size distribution withaverage grain sizes of less than or equal to about 15 micrometers. Inone embodiment, the articles have a monomodal grain size distributionwith average grain sizes of about 1 micrometer to about 15 micrometers.In another embodiment, the articles have a monomodal grain sizedistribution with average grain sizes of about 5 micrometers to about 10micrometers. In yet another embodiment, the articles have a monomodalgrain size distribution with average grain sizes of about 1 micrometerto about 5 micrometers.

They display a biaxial flexural strength of greater than or equal toabout 300 MPa and display a specular transmittance of greater than orequal to about 75% between 632 and 800 nm wavelength for an articlehaving a thickness of about 9.6 millimeter. This allows for betterperformance of the spinel material as transparent armor and in radomes.In one embodiment, the 9.6 millimeter thick spinel article displays aspecular transmittance of greater than or equal to about 78%. In anotherembodiment, the 9.6 millimeter thick spinel article displays a speculartransmittance of greater than or equal to about 80%. In yet anotherembodiment, the 9.6 millimeter thick spinel article displays a speculartransmittance of greater than or equal to about 81.5%.

The spinel article displays a biaxial flexural strength of greater thanor equal to about 300 MPa. In one embodiment, the spinel articledisplays a biaxial flexural strength of greater than or equal to about350 MPa. In another embodiment, the spinel article displays a biaxialflexural strength of greater than or equal to about 400 MPa. In yetanother embodiment, the spinel article displays a biaxial flexuralstrength of greater than or equal to about 480 MPa.

The following example illustrates methods and embodiments in accordancewith the invention, and as such should not be construed as imposinglimitations upon the claims. These examples demonstrate the manufactureof the spinel material described herein and demonstrate its performancecompared with other spinels that are commercially available.

EXAMPLES

The example was conducted to demonstrate how the spinel disclosed hereinis manufactured and tested.

Spinel powder having the composition MgAl₂O₄ and having a purity ofgreater than 99.98% was calcined in air at 700° C. for 3 hours. Thepowder was dispersed in a milling medium comprising 3 weight percent(with respect to the solid) of citric acid neutralized with ammoniumhydroxide to a pH of 4 to 5. The powder was present in the millingmedium in an amount of 15 volume percent. Lithium in an amount of 10ppm, with respect to the powder weight, was added to the milling mediumas a sintering aid. The lithium was added to the milling medium in theform of lithium hydroxide. Polyethylene glycol was added as a binder inan amount of 1.5 wt %, based on the weight of the powder.

The milling medium with the powder contained therein was subjected toball milling. The ball milling was performed overnight for a totalperiod of 15 hours. Polymeric particles formed during the milling wereremoved via filtration through a membrane. The membrane had an eightmicrometer nominal pore size. The suspension was freeze granulated atthe boiling temperature of liquid nitrogen. The resulting granulatedpowder was sieved through a 60 mesh sieve to produce the powder used infurther processing. The granulated powder was first pressed uniaxiallyat approximately 21 MPa and then pressed isostatically at a pressure ofapproximately 280 MPa to produce an article of a desired shape. Severaldisc shaped samples (hereinafter “samples”) were prepared in thismanner.

The samples were burned out in an air/oxygen mixture over the course of3 days to a temperature of 900° C. The heating profile to reach thetemperature of 900° C. is as follows: the samples are heated at 1°C./min to 50° C. where they are held for 2 hours, 1° C./min to 170° C.(not held at 170° C. for any period of time), 0.2° C./min to 235° C.(not held at 235° C. for any period of time), 0.1° C./min to 270° C.(held at 270° C. for 5 hours), 0.1° C./min to 275° C. (held at 275° C.for 7 hours), 0.1° C./min to 350° C. (not held at 170° C. for any periodof time), 0.8° C./min to 400° C. (held at 400° C. for one hour), 1°C./min to 600° C. (held at 600° C. for 1 hour), 2° C./min to 900° C.(held at 900° C. for 8 hours), 5° C./min to room temperature. Theburned-out samples were sintered to 1260° C. for 3 hours and then hotisostatically pressed to 1450° C. in pressurized argon having a pressureof 45 ksi. Following the hot isostatic pressing and polishing, thesamples had a final thickness of 9.6 millimeter.

FIG. 2 shows the specular transmittance of a 9.6 millimeter thick samplemeasured for electromagnetic radiation of wavelength in the 500nanometers to 900 nanometers range. The sample exhibited a transmittanceof 81.5% at 632 nanometers. One of the 9.6 millimeter thick discs wascut in half across the diameter and the cut section was then ground,polished, and thermally etched for microstructural characterization byscanning electron microscopy. FIG. 3 shows a scanning electronmicrograph taken from a representative area of the disc. It can be seenthat the spinel material has a unimodal grain size distribution with anaverage grain size in the range of 1 to 2 micrometers.

Another 9.6 millimeter thick disc was cut up as follows to producesamples that were used to measure the strength of the spinel materialusing the ring-on-ring test. Several 14 millimeter diameter discs werecut out in a square form, then sliced to 1.5 to 2 millimeter thickpieces that were then glued together and turned to a cylindrical shapeof 14 millimeter diameter. The glue was then removed and the discs wereground on both surfaces to reduce the thickness and improve the surfacefinish. All the discs were then polished on one side to a mirror finishwith a final thickness of 0.8 millimeters. A total of 20 discs weresubjected to the ring-on-ring test. FIG. 4 shows the Weibull plot of theflexural strength data measured from these samples. The average flexuralstrengths measured from these samples was 487 MPa.

The flexural strength measured in these samples was far superior whencompared to what has been reported for various spinel materials in theliterature. In order to validate this difference, a state-of-the-artspinel material that was prepared by hot pressing and hot isostaticpressing was acquired from a vendor. Disk samples for ring-on-ringflexural testing were prepared in the same manner as the newly disclosedmaterial per the description above. It is to be noted that all of theflexural test samples used in the comparison were ground and polished bythe same vendor using the same equipment and employing the sameprocedure for all samples. This is to be noted because surface flawsthat are introduced during sample preparation could have a direct effecton the measured strength. The flexural strength data measured from thenewly disclosed spinel samples is compared with what is measured fromthe vendor material samples in FIG. 5. The mean strength measured fromthe disclosed spinel samples was 487 MPa compared to a mean strength of142 MPa measured for the vendor spinel samples. This result clearlyillustrates that the disclosed material does offer a substantialstrength improvement over state-of-the-art material.

The method of processing described herein ensures that there areessentially no voids. It facilitates the achieving of a density that isabout 99.999 to about 99.99995 volume percent of the theoretical valueof density that can be achieved for a spinel.

While the invention has been described in detail in connection with anumber of embodiments, the invention is not limited to such disclosedembodiments. Rather, the invention can be modified to incorporate anynumber of variations, alterations, substitutions or equivalentarrangements not heretofore described, but which are commensurate withthe scope of the invention. Additionally, while various embodiments ofthe invention have been described, it is to be understood that aspectsof the invention may include only some of the described embodiments.Accordingly, the invention is not to be seen as limited by the foregoingdescription, but is only limited by the scope of the appended claims.

1. An article comprising: a spinel material, wherein the spinel materialhas a monomodal grain size distribution with average grain sizes of lessthan or equal to about 15 micrometers, and a biaxial flexural strengthof greater than 400 megapascals when measured by a ring-on-ring flexuraltest as per ASTM Standard C1499-08.
 2. The spinel article of claim 1,wherein the spinel comprises MgAl₂O₄, MgGa₂O₄, ZnAl₂O₄, ZnGa₂O₄,SiMg₂O₄, ZnAl₂O₄, (Fe,Mn,Zn)(Fe,Mn)₂O₄, (Fe.Mg)Cr₂O₄, Fe₃O₄, FeAl₂O₄,TiFe₂O₄, MnFe₂O₄, NiFe₂O₄, SiMg₂O₄, or a combination comprising at leastone of the foregoing spinels.
 3. The spinel article of claim 2, whereinthe article has a specular transmittance of greater than or equal toabout 75% in the wavelength range from 632 to 800 nm for a sectionhaving a thickness of about 9.6 millimeter.
 4. The spinel article ofclaim 3, wherein the spinel comprises MgAl₂O₄, MgGa₂O₄, ZnAl₂O₄,ZnGa₂O₄, SiMg₂O₄ or a combination comprising at least one of theforegoing spinels.
 5. The spinel article of claim 1, wherein the articlehas a specular transmittance of greater than or equal to about 80% inthe wavelength range from 632 to 800 nm for a section having a thicknessof about 9.6 millimeter.
 6. The spinel article of claim 1, wherein thearticle has a biaxial flexural strength of greater than 400 megapascalsand a specular transmittance of greater than or equal to about 80% inthe wavelength range from 632 to 800 nm for a section having a thicknessof about 9.6 millimeter; the biaxial flexural strength being measured bya ring-on-ring flexural test as per ASTM Standard C1499-08.
 7. A spinelarticle manufactured by a method comprising: calcining a spinel powder;milling the powder in a milling medium; filtering the powder to removeorganic particles; granulating the powder; screening the powder to amesh size of less than or equal to about 40 to about 200 mesh; pressingthe powder to form an article; burning out organics from the article;sintering the article; and hot isostatically pressing the article; andwherein the spinel article has a biaxial flexural strength of greaterthan 400 megapascals when measured by a ring-on-ring fleural test as perASTM Standard C1499-08.
 8. The spinel article of claim 7, wherein thespinel powder comprises MgAl₂O₄, ZnAl₂O₄, (Fe,Mn,Zn)(Fe,Mn)₂O₄,(Fe.Mg)Cr₂O₄, Fe₃O₄, FeAl₂O₄, TiFe₂O₄, MnFe₂O₄, NiFe₂O₄, SiMg₂O₄, or acombination comprising at least one of the foregoing spinels.
 9. Thespinel article of claim 7, wherein the spinel powder prior to calcininghas a surface area of about 20 to about 60 square meters per gram and anaverage particle size of about 10 to about 200 nanometers.
 10. Thespinel article of claim 7, wherein the article has a biaxial flexuralstrength of greater than 400 megapascals and a specular transmittance ofgreater than or equal to about 80% in the wavelength range from 632 to800 nm for a 9.6 millimeter thick article after the hot isostaticpressing and polishing; the biaxial flexural strength being measured bya ring-on-ring flexural test as per ASTM Standard C1499-08.
 11. A methodcomprising: calcining a spinel powder; milling the powder in a millingmedium; filtering the powder to remove organic particles; granulatingthe powder; screening the powder to a mesh size of less than or equal toabout 40 to about 200 mesh; pressing the powder to form an article;burning out organics from the article; sintering the article; and hotisostatically pressing the article.
 12. The method of claim 11, whereinthe calcining is conducted at a temperature of about 600 to about 900°C.
 13. The method of claim 11, wherein the milling is accomplished viaball milling, attrition milling, vibratory milling and/or jet milling.14. The method of claim 11, wherein the milling is substituted byultrasonication.
 15. The method of claim 11, wherein the milling mediumis a solution or a suspension.
 16. The method of claim 15, wherein thesolution comprises a liquid, a dispersant, a binder, an optionalplasticizer and metal salt, where the metal is from Group I of theperiodic table.
 17. The method of claim 16, wherein the liquid is wateror an organic solvent.
 18. The method of claim 16, wherein thedispersant is an acid, a pre-neutralized acid or a base, and wherein thedispersant is added to the milling media in an amount of about 1 toabout 5 weight percent, based upon the total weight of the powder. 19.The method of claim 18, wherein the acid is citric acid, succinic acid,polyacrylic acid, acetic acid or a combination comprising at least oneof the foregoing acids.
 20. The method of claim 18, wherein thepre-neutralized acid is diammonium citrate, ammonium citrate tribasic,ammonium polyacrylate, or a combination comprising at least one of theforegoing pre-neutralized acids.
 21. The method of claim 18, wherein thebase is ammonium hydroxide, tetramethyl ammonium hydroxide, or acombination comprising at least one of the foregoing bases.
 22. Themethod of claim 16, wherein the binder is an organic polymer.
 23. Themethod of claim 16, wherein the organic polymer is polyacrylamide,polyvinylalcohol, polyvinylpyrrolidone, polyethylene glycol,hydroxyethylcellulose, methylcellulose, polyethyleneimine, or acombination comprising at least one of the foregoing organic polymers.24. The method of claim 16, wherein the binder is used in an amount ofabout 0.5 to about 5 weight percent, based upon the total weight of thepowder.
 25. The method of claim 16, wherein the plasticizer is glycerin,ethylene glycol, tetraethylene glycol, dibutyl phthalate, or acombination comprising at least one of the foregoing plasticizers andwherein the plasticizer is generally used in an amount of about 0.1 toabout 5 wt %, based on the total weight of the powder.
 26. The method ofclaim 16, wherein the metal salt is used as a sintering aid; the metalsalt comprising nitrates, hydroxides, oxalates, carbonates,acetylacetonates, and acetates of Group I metals.
 27. The method ofclaim 11, wherein the ratio of the powder to the milling medium is about10 to about 25 volume percent.
 28. The method of claim 11, wherein thepressing the powder to form an article is conducted using an isostaticpressure of about 140 megapascals to about 420 megapascals.
 29. Themethod of claim 11, wherein the sintering of the article is conducted ata temperature of about 1200° C. to about 1350° C. for a time period ofabout 0.5 hours to about 24 hours.
 30. The method of claim 11, whereinthe article is subjected to hot isostatic pressing to a temperature ofabout 1300 to about 1600° C. and a pressure of about 15 to about 60kilograms per square inch.